1:24 Because of the union of believers with Christ, Paul's sufferings for the sake of the church can be called Christ's afflictions as well.
1:25 the stewardship, assignment, office (1 Cor. 4:1).
1:26-27 the mystery. The secret unknown in OT times but known now only by divine revelation of the indwelling of Christ. See note on Eph. 3:3
1:28 Complete. Mature. The word was used by the mystery religions to designate those who had been initiated into the "secrets" of those religions. In Christ all can have wisdom and maturity.
The Ryrie Study Bible [New American Standard Bible], p. 1476